poor baby... i have a 6 year old baby and i can't think of this ever happening to me... i think i would go out on the street and look for clue by clue even if i had to go into houses and check everything... in this case i think there are several possible options
someone had kidnapped her while she was passing through the neighborhood which would have been very quick and well done...
she ran away... although this is not confirmed because i haven't seen many sources that explain it, she was talking about running away... it is possible that as soon as her brothers were out of sight, she left... however there are simply no footprints or any other known evidence of it... also, how far could her six year old self have gotten before something went wrong?
I also read that her parents were divorced (but this is not confirmed) and lived in separate houses... it is possible that one of the parents kidnapped her as an act of revenge or out of jealousy...
or the neighbors kidnapped her...
and again the overconfidence of the north americans... a 6 year old baby should never be alone, always in sight of the parents...
I hope baby that one day it will be known what happened to you and that your final moment was not painful...
whatever the case
rest in peace crystal